The present invention relates to a rear wheel steering control system.
A rear wheel steering system is useful to provide a flat characteristic of a vehicle yaw rate versus a steering frequency of front wheels, and to produce a yaw rate in proportion to a steering angle .theta. independent of a steering speed with no phase lag.
One conventional example of such a rear wheel steering system is disclosed in Japanese Patent Provisional Publication No. 60-229873. This rear wheel steering system is arranged to steer the rear wheels so that the rear wheel steer angle .delta..sub.r is made equal to the algebraic sum of a proportional term obtained by multiplying the steering angle .theta. by a positive proportional constant K, and a first derivative term obtained by multiplying a steering angular speed .theta. by a negative first derivative constant .tau..sub.1 (.delta..sub.r =K.theta.+.tau..sub.1 .theta.).
When the steering angle .theta. is varied as shown in FIG. 7, this control system varies the proportional control quantity K.theta. and the derivative control quantity .tau..sub.1 .theta. as shown by a dotted line and a one dot chain line in FIG. 7. As a result, the rear wheel steer angle .delta..sub.r is controlled as shown by a solid line a in each of FIGS. 7 and 8.
The control constants K and .tau..sub.1 are so determined as to obtain a flat characteristic of the yaw rate gain. Therefore, an opposite-phase steering period .DELTA.T.sub.1 during which the rear wheels are steered in the opposite direction to the steering direction of the front wheels is considerably long as shown in FIG. 8. This causes passengers of the vehicle to feel, at the beginning of a turn, the unnatural feeling that the rear end of the vehicle is swung in the direction opposite to the centripetal direction, and degrades riding comfort.
It is possible to meet this problem by decreasing the first derivative constant .tau..sub.1. In this case, the rear wheel steer angle is controlled as shown by a dotted line b in FIG. 8, and the opposite-phase steering period is reduced from .DELTA.T.sub.1 to .DELTA.T.sub.2. However, the amount of the opposite-phase steering is decreased from -.delta..sub.r1 to -.delta..sub.r2 as shown in FIG. 8, and accordingly the response characteristic of the yaw is deteriorated.